Among the opportunistic infections in AIDS patients are those caused by two related parasites: Toxoplasma gondii and Cryptosporidium parvum. Both organisms belong to the phylum Apicomplexa, whose members (including the causative agent of malaria) have recently been shown to contain a unique plastid-like organelle. Because humans do not have a homologous organelle, the plastid may provide a unique target for the development of drugs that act specifically against these parasites. However, little is known about the protein constituents of this plasmid or its function. T. gondii is the only apicomplexan parasite for which genetic approaches are well-developed. This proposal describes a plan to examine the localization of proteins to the apicomplexan plastid using T. gondii as our model. We will obtain the sequence of the entire coding region of a cDNA predicted to specify a nuclearly encoded plastid protein. We will then verify that the protein is indeed a plastid resident, using an epitope tagging/transfection approach. Finally we will determine if sequences from the plastid protein can route a reporter molecule to the plastid. These studies will provide the first information about the biogenesis of the apicomplexan plastid and lay the groundwork for studying plastid function.